1. Field of the Invention
The present invention relates generally to color sensors and specifically to a method and apparatus for employing a light shield to modulate a pixel's color responsivity.
2. Description of the Related Art
Imaging devices can typically employ a sensor (not shown) to detect light and, responsive thereto, generate electrical signals representing the light. A sensor typically includes a light sensing element (e.g., a photodiode), and associated circuitry for selectively reading out the electric signal provided by the light sensing circuit. The light sensing circuit operates by the well-known photoelectric effect that transforms light photons into electrons that constitute an electrical signal.
Color imaging devices employ color filter arrays (herein referred to as CFAs) to generate color output. CFAs include a plurality of CFA elements that typically include red, green and blue elements.
FIG. 1 illustrates a perspective view of a conventional imaging device 2 that includes an IR blocking filter 4, a lens assembly 5, and an imager and package 6. The imager and package 6 includes a pixel array 7 having a substrate with an active area 8 and a color filter array 9 disposed thereon.
A CFA element 9 is overlaid on the substrate 8 and covers the light sensing circuit. The combination of the sensor with the corresponding CFA element is often referred to as a pixel. For example, if a red CFA element is overlaid over a light sensing circuit, that pixel is referred to as a red pixel. Similarly, if a green CFA element is overlaid on a light sensing circuit, that pixel is referred to as a green pixel.
There are two primary types of imagers. First, there are those imagers employing CCD (charge coupled device) technology. Second, there are imagers that are made using complementary metal oxide semiconductor (CMOS) processes.
One common problem associated with the use of color filter arrays on CCD and CMOS imagers is that pixel/sensor responsivity varies with the specific type of color.
Generally, the responsivity of a pixel of a first color is different than the responsivity for a pixel of a second color. For example, in a system employing a red color pixel, a green color pixel and a blue color pixel, assuming a uniform integration time (that is, the time of exposure to light being equal), and a typical scene being imaged, the signal to noise (S/N) ratio of the pixels win not be equal due to differing responsivity between the pixels. Blue pixels typically have the least responsivity; consequently, the signal to noise ratio of the blue pixels is less than the signal to noise ratio of the red and green pixels.
Moreover, in capturing an image with equal amounts of red, green and blue light, the pixels having the greatest sensitivity (typically the red and green pixels) saturate first. Specifically, the storage capacitors associated with the red and green pixels reach a maximum capacity of stored photoelectrons (i.e., saturate) before the blue pixels.
Once a pixel saturates, the exposure to light is stopped (by closing a mechanical shutter, for example) to avoid blooming and other saturation artifacts. Blooming is simply a false electrical signal representation of light intensity at a neighboring pixel because of charge leakage from the saturated pixel.
However, stopping the exposure, although preventing blooming and other saturation artifacts, compromises the signal to noise ratio of the pixels with the lowest sensitivity to light (typically the blue pixels). The consequence of stopping the exposure when the red and green pictures are saturated, is that the pixels with the lowest sensitivity (typically the blue pixels) suffer in signal to noise ratio.
Prior art sensors do not compensate for color responsivity variation among the different color pixels. Accordingly, when an exposure is made, the exposure time is adjusted to avoid saturation in the most sensitive pixels. Thus, as a result, blooming is avoided in neighboring pixels. The result of this adjustment in exposure time is a degraded signal to noise (S/N) ratio in the least sensitive pixels (typically the blue pixels).
A conventional approach is to increase the signal to noise ratio of the blue pixels by increasing the integration time (i.e., the exposure time). However, as one increases the exposure time, although the signal to noise ratio of the blue pixels is increased, the red and green pixels saturate and are subject to undesirable saturation artifacts (these undesirable artifacts are commonly referred to in the art as blooming). To counteract the saturation artifacts, the prior art employed anti-blooming mechanisms in the pixels. However, these mechanisms increase the cost and complexity of the color pixels. Moreover, these anti-blooming mechanisms are ineffective to eliminate the blooming effect while still obtaining a desired increase in the signal to noise ratio of the blue pixels.
Accordingly, there remains an unmet need in the industry for a method and apparatus that modifies the responsivity of a color pixel to overcome the disadvantage discussed above.